Two Tamil Tiger aircraft launched a ­surprise bomb raid on the Sri Lankan capital last night, hitting a tax building in Colombo and injuring at least 27 people.

The attack amounts to a major embarrassment for Sri Lanka's government, which had claimed to have destroyed all the Tamil Tiger rebels' hidden runways and rendered their rudimentary air squadron useless in a recent offensive that has driven the insurgents from most of their northern strongholds.

Shortly after the bombing, Sri Lanka's air force gave chase to two rebel light ­aircraft and shot down one of them, military spokesman Brigadier Udaya Nanayakkara said.

He said that the wreckage and the body of the pilot were recovered

and power was cut off to the city as searchlights scanned for signs of more rebel planes.

Anti-aircraft fire lit up the night sky. Witnesses at the international airport north of the capital said they heard a loud explosion and anti-aircraft fire, but the airport had apparently not come under attack. Last night's attack – the first since ­October – shows that the insurgents still pose a threat despite a big army offensive that has pushed them out of nearly all their strongholds in the northern part of the island.

The rebels' main fighting force is now boxed into a small strip of land in the north, and fierce fighting continues.

During the offensive, casualties had spiralled, with both sides accused of targeting civilians. Health officials and witnesses have accused the government of launching artillery barrages into areas crowded with civilians, while also accusing the rebels of shooting civilians who tried to escape. Confirmation of the allegations is not possible because independent journalists are barred from the area.

The violence has led to a "dramatic increase" in civilian casualties, with ­independent monitors estimating 2,000 civilians have been killed and another 5,000 wounded over the past month, Human Rights Watch said in a report today.

The New York-based rights group called on both sides to immediately stop "the ongoing slaughter of civilians". The government has repeatedly denied causing any civilian deaths. However, government official Rajiva Wijesinha told reporters some "collateral damage" may have occurred, but he said Human Rights Watch's numbers were exaggerated.

The rebels have denied holding civilians as human shields or shooting at those who flee. Reuters